In a typical HVAC system, refrigerant passes through a cycle. Refrigerant goes from a vapor to a liquid in a condenser (heat exchanger) as heat is lost to air passing outside the condenser, blown by a fan. The refrigerant then goes through an expansion valve which lowers the pressure. The refrigerant next passes through an evaporator, which converts the refrigerant from a liquid to a vapor as heat is received from air blown past the evaporator (heat exchanger) by a fan. The vapor refrigerant will then pass through a compressor which raises the pressure of the refrigerant, and then the refrigerant returns to the condenser.
A condenser can comprise a plurality of tubes exposed to air blown by a fan. The tubes can be divided into multiple groups or stages. For example, there can be three sets of tubes comprising first, second and third stages. The stages can be activated or closed depending on the system load. Typically there is a compressor associated with each stage. Often, between the stages there will be a “dead tube” that marks the boundary between each stage. Near the dead tube, or any frontier between stages, there can exist temperature or pressure differences between stages, causing strain in the condenser.